[Feat]Copy-free save and load for cuckoo hashtable

[Lifann]

Description

Brief Description of the PR: Since dynamic embedding could be super large for memory limit. save and load with traditional TensorFlow checkpoint mechanism will use a lot of memory when saving or loading. This PR provides a method to save or load files for dynamic embedding tables, without full volume copying.

Type of change

• [x] Bug fix
• [ ] New Tutorial
• [ ] Updated or additional documentation
• [x] Additional Testing
• [x] New Feature

Checklist:

• [x] I've properly formatted my code according to the guidelines
  • [ ] By running yapf
  • [ ] By running clang-format
• [ ] This PR addresses an already submitted issue for TensorFlow Recommenders-Addons
• [ ] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works

How Has This Been Tested?

Yes

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[acmore]

This feature is very useful. Looking forward to it.

May I ask how to use it? In our case, we will use estimator and save checkpoints per epoch. So should we customize a saver to save the tables manually? But if so, how can we get all the tables to save?

Currently it only support usage like save_op = table.save(path). In eager mode, it's pythonic and simple. In graph mode, it should be managed on graph, like SessionRunHook or run sub-branch of graph.

[PWZER]

This is a good solution, we want to solve this problem too, When can it be merged?

[MoFHeka]

Try this: https://github.com/tensorflow/recommenders-addons/blob/master/docs/api_docs/tfra/dynamic_embedding/FileSystemSaver.md